Trace levels of copper, as well as other metals, are known to cause failures in integrated circuits (ICs) and optoelectronic devices such as optoelectronic integrated circuits (OICs). These so-called heavy metals act as effective recombination centers and increase leakage when present in the vicinity of p-n junctions.
During the manufacture of ICs and OICs, copper and other trace metal contaminants may originate from raw materials and gases, sputtering targets, and cleaning solutions, as well as from the processing equipment used in device manufacture. These trace metals may contaminate wafer surfaces, dielectric films, or metallization layers. Often, the detection and quantification of such trace impurities by conventional analytical techniques are complicated or defeated by the presence of very high levels of metals that are intended to be incorporated in the device structure. These intended metals are referred to here as matrix elements, and the unwanted impurity metals as trace contaminants.
In conventional IC manufacture, low levels of contaminants are detected or measured using a variety of well established techniques. A typical method is to dissolve the material being investigated, or in the case of device structures partially fabricated, treat the surface of the structure with a solvent, then analyze the solution by spectrometry. An effective, and widely used, spectrometric technique is Inductively Coupled Plasma Mass Spectroscopy (ICP-MS). This is a quantitative elemental mass spectrometric technique that offers the requisite sensitivity for detecting very low levels of impurities. In some cases matrix effects arise using this technique but in general meaningful data on the trace metallic content of the materials analyzed can still be extracted.
However, when analyzing titanium metal films (after dissolution) by ICP-MS, there is major matrix interference from the titanium ions that precludes effective detection of low levels of copper. This is due to the spontaneous oxidation of titanium and the formation of oxides of isotopes of titanium, i.e. Ti.sup.47 O.sup.16 and Ti.sup.49 O.sup.16. The masses of these oxides are essentially equivalent to those of the isotopes of copper, i.e. Cu.sup.63 and Cu.sup.65, and cannot be differentiated by quadrupole mass spectrometry (they are spaced by only 0.017 amu and 0.015 amu, respectively). High resolution mass spectrometry using more expensive tools, e.g. ICP-MS instruments, may be able to discriminate the small differential, but very high resolution settings would be required, resulting in a concomitant loss of sensitivity. This mass spectral interference problem is general, affecting not only ICP-MS, but also SIMS (Secondary Ion Mass Spectrometry) and GDMS (Glow Discharge Spectrometry) techniques.